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PHYSICAL REVIEW B 84, 245310 (2011)

Perpendicular magnetic anisotropy and high spin-polarization ratio in epitaxial Fe-N thin films Nian Ji,1,2 M. S. Osofsky,3 Valeria Lauter,4 Lawrence F. Allard,5 Xuan Li,1 Kevin L. Jensen,3 Hailemariam Ambaye,4 Edgar Lara-Curzio,5 and Jian-Ping Wang1,2,* 1

The Center for Micromagnetics and Information Technologies and Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, USA 2 Department of Physics, University of Minnesota, Minneapolis, Minnesota 55455, USA 3 Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, USA 4 Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 5 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA (Received 27 June 2011; revised manuscript received 7 September 2011; published 14 December 2011) We have demonstrated a general framework for realizing and modulating perpendicular magnetic anisotropy in a rare-earth-element and heavy-metal-free material system. Using GaAs(001)/Fe(001) template, we have developed a synthesis scheme to produce epitaxial body center tetragonal Fe-N with (001) texture. By varying the N doping concentration, the crystal tetragonality (c/a) can be tuned in a relatively wide range. It is found that the Fe-N layer developed a strong perpendicular magnetic crystalline anisotropy (MCA) as it approaches the iron nitride interstitial solubility limit. Further annealing significantly improves the MCA due to the formation of chemically ordered Fe16 N2 . In addition to realizing an MCA up to 107 erg/cm3 , the spin polarization ratio (P ∼ 0.52), as probed directly by a point-contact Andreev reflection (PCAR) method, even shows a moderate increase in comparison with normal metal Fe (P ∼ 0.45). These combined properties make this material system a promising candidate for applications in spintronic devices and also potential rare-earth-element free magnets. DOI: 10.1103/PhysRevB.84.245310

PACS number(s): 75.25.−j

I. INTRODUCTION

Giant magnetoresistive devices with perpendicular magnetic anisotropy have drawn increased attention due to their potential for higher storage densities in magnetic memory applications.1–3 To fabricate such devices, one essential component, the ferromagnetic electrode, which serves as a polarized electron source, has to satisfy two criteria: (1) high value of spin polarization and (2) magnetic easy axis points out of the film plane. The focus of most efforts has been on transition metal (TM) and heavy-elements-based multilayers, which are proven structures with high perpendicular magnetic anisotropy (PMA). However, concerns, such as large damping due to the heavy elements and poor compatibility with the common barrier material, prevent them from being realized in practical applications. For magnetic tunnel junction (MTJ) devices, the recently developed ultra-thin FeCoB layer with PMA offers a new route to tackle the problem, in which a high tunneling magnetoresistance (TMR) (>100%) and an easy axis perpendicular to the film plane are simultaneously realized.4 However, the utilization of interface anisotropy that produces out-of-plane easy axis demands precise and critical control of layer thicknesses (10%) as the N interstitial solubility increases, given the progressively higher N2 partial pressure [Fig. 1(d)] used in depositing these samples. To further examine the crystal quality and epitaxial relationship of the present films, we used high-resolution (HR) transmission electron microscopy (TEM) (JEOL 2200FS) to study a cross-sectional sample that may possess the largest c lattice constant according to the XRD. Figure 2(a) shows a high-angle annular dark-field TEM image with low magnification. In addition to the GaAs substrate, Fe and Fe-N layers, as outlined in the figure, Cr cap layer (∼8 nm), and carbon (∼1 μm) layer are deposited ex situ for protection from further oxidation. The brightness contrast between two layers is clearly seen, which is due to the presence of the light element N in the Fe-N layer. Aberration-corrected lattice images with nominal resolution of